Abstract
This paper investigates the experiment work of laser diode coherence properties which are the most fundamental property. In this experiment a scientific instrument designed which called Michelson interferometer to measure a beam of light that splitting it into two equal halves by using a beam-splitter which one of the beams called reference beam and the second beam travels an extra distance, so it gets slightly out of the phase. the output power is measured by photo-detector as a function of the input current in order to determine threshold current, because of the laser diode is sensitive to temperature, which it set to25°C, the measured results was above the threshold current because laser diode